Internet Protocol Television or IPTV is a term used to refer to the delivery of television and video (collectively referred to below as “TV”) over IP-based communication networks. IPTV is in fact a collection of protocols including compression, playout, and distribution protocols. In terms of distribution across networks, three classes have been defined, namely multicast, unicast, and peer-to-peer distribution.
Of these, it is envisaged that multicast will be used mainly for live-streaming of TV. Multicast relies upon the IETF defined Internet Group Management Protocol (IGMP) to create and change multicast receiver groups at multicast agents within an IP network. Multicast agents receive TV and multicast it onwards to group members. Multicast is a very effective mechanism for transporting a set of TV channels to a large number of viewers. Multicast is not however a suitable distribution mechanism for so-called “time-lapse” TV, where a viewer can pause a live-TV program and then resume watching it at a later time. Also, the efficiency gained by using multicast decreases with the number of viewers.
Unicast distribution typically relies upon the Real Time Streaming Protocol (RTSP) which allows a client to remotely control a streaming media server, issuing VCR-like commands such as “play” and “pause”, and allowing time-based access to files on a server. Unicast creates individual sessions for each connected user, so the same content will get transferred once per user over the network. Since each session is individual, pausing, resuming, rewinding and forwarding can easily be implemented when unicast is used. Unicast is a suitable method for Video-on-Demand, time-lapse TV and similar (more personal) TV services. A disadvantage of unicast distribution is that it consumes a large amount of bandwidth for live-TV.
Peer-to-peer distribution (also know as “P2P”) uses each client and/or node in a network for the distribution of media. When a program is viewed by a given client/node, different parts of the program can be downloaded from different neighbouring clients/nodes. Network operators might want to implement restricted P2P distribution within their own networks, with content being cached at multiple nodes within a given network for onward transfer to clients. Clients may or may not be able to share content between each other. Peer-to-peer has a lot of advantages and lies somewhere in between unicast and multicast when it comes to bandwidth usage. Whilst peer-to-peer distribution can be less bandwidth efficient that multicast, it does offer the possibility of time-lapse play even for live TV. Of course, peer-to-peer distribution is not so satisfactory for live TV as significant delays result from the need for different nodes to cache and forward different parts of a program.
The decision as to which distribution mechanism to use for IPTV is therefore very much dependent upon the service to be delivered and the network capabilities/performance, as well as upon the quality of service (QoS) acceptable to the viewers. Distribution mechanism selection is also important when it comes to services other than IPTV, such as gaming.
Existing applications are usually designed for a single distribution method. There is no flexibility for example for a set top box to choose whether to receive a TV feed via a multicast or unicast channel, let lone switch in the middle of the feed from one method to another in an attempt to optimize performance.
It is desirable to allow one or both of the source or destination of IPTV (or other media) to select the distribution mechanism according to prevailing conditions and requirements. Moreover, it is desirable to achieve this in such a way that media can be distributed over different legs of a transmission path using different distribution mechanisms, and so that the mechanism can be changed during a given session.
Another very complex problem with media today is that operators are not allowed to cache all content due to different licensing restrictions for different media. This is typically solved by selecting multicast for live-TV (no caching) and unicast for video-on-demand (caching). The result is wastage of bandwidth in the network due to some media clips being uncachable.
Media distribution schemes are described in US2005/0281270 and US 2003/0051251.